Product Review: Auto-Vac Bagging System Plus
Dateline: 08/25/97I recently purchased a new piece of equipment for my home shop: the Auto-Vac Bagging System Plus from Aerospace Composite Products (ACP). This is a small vacuum pump with an automatic shut-off (hence the Auto-Vac). The kit comes with some bagging materials (Bagging System), and I purchased it with some optional bagging materials (Plus). Total price for my system was $275.00 plus $13.25 for shipping.
ACP sells several vacuum pumps and vacuum kits. Their three pumps are the EZ-VAC I, the EZ-VAC II, and the AUTO-VAC. According to the ACP catalog, the EZ-VAC I can pull 7" Hg and the EZ-VAC II 24" Hg. The AUTO-VAC is an EZ-VAC II with an adjustable valve to shut off the pump at a target pressure. All pumps are available in either 115 Volt or 230 Volt versions. The higher voltage increases the pumping speed, but not the ultimate pressure.
The bagging kits include a pump, neoprene hose (1/8 inch diameter?), bagging, breather cloth, Quick Lock seals, and a vacuum connector. The Plus kits also include mylar, peel ply, and a paste mold release. All kits come with instructions for operating the pump and for making a foam-core wing. The wing layup instructions are useful for learning about composite layups in general, and not just for running the pump.
My kit arrived about 4-5 weeks after I placed the order (this was well before the UPS strike). It arrived in two boxes--one for the pump, the other for the bagging materials. Everything arrived without damage, but the vacuum connector is so small that it almost got lost in the packing material. It would be better if this piece were taped to the pump or some other, larger package.
The pump is small (5 or 6 inches long and 4 inches in diameter) and mounted on a wooden board. The board is painted with a granite-like coating and has routed edges, so it looks rather nice. The pump is rated at 1050 cubic inches per minute (less than 1 cubic foot per minute); I have no way of verifying the speed, but I did run some pump down tests (see below). The pump seems a little noisy, but isn't annoying. There were no obvious places for checking or adding oil, and the instructions say nothing about pump maintenance.
The pump is connected, via a neoprene tube, to a vacuum reservoir (also mounted to the board). The reservoir is a plastic tube about 3-4 inches in diameter and 8-10 inches long. A dial gauge and the auto shut-off valve are mounted to the reservoir; another neoprene tube leading from the reservoir is used to pull vacuum. The reservoir helps reduce fluctuations in pressure and serves to protect the pump from resin (though there appears to be no way to open the reservoir to clean it out).
The vacuum connector looks like a miniature version of the through-bag connectors used with larger pumps. There is no quick-disconnect; you simply slide the neoprene tubing over the connector. Unfortunately, this means you can't remove the hose easily to vent the bag. Also, there is no bleed valve, so the only way to vent is to pull off the hose, unscrew the connector, or break the bag seal.
The bag is actually a tube bag (18 inches wide, though wider versions are available). It looks like it is made from the same material as ordinary (low-temp) vacuum bag, but it is a double ply, closed on the long edges (10 feet) and open on the short edges (18 inches) to form a long tube.
You seal the bag using the Quick Lock seals. Each seal consists of a plastic rod and C-channel. You place the open end of the bag between the rod and channel, then snap the rod into the channel. I didn't think this would make a good seal, because it was difficult to keep the bag from wrinkling in the seal, but my leak test (see below) proved otherwise. In fact, I was somewhat skeptical of this bagging system, but came to like it after giving it a try.
After unpacking everything, I started running some tests. A label on the reservoir said the auto shut-off valve was preset for 7" Hg. I started the pump and held the open end of the hose against my thumb. Pump-down time was 15 seconds, but the shut-off pressure was 10" Hg. After releasing my thumb, the pump restarted at 7" Hg.
At all pressure settings, the pump would switch on again after a drop of 3" Hg. The auto on/off feature is very nice, but I would like the drop to be adjustable. 3" Hg is nearly 50% of the standard pressure setting, which I would usually consider too much of a variation in the cure cycle.
The auto shut-off valve is adjusted by turning a set screw. The screw is covered by a small metal cap that snaps off (I almost lost the cap several times). The screw isn't marked, so you have to run some tests to calibrate it. I pumped down the reservoir to 12" Hg (25 seconds), 14" Hg (39 seconds), and 15" Hg. The maximum vacuum I could get was just over 15" Hg; at that level, the pump never shut off.
At all pressure settings, the pump would switch on again after a drop of 3" Hg. The auto on/off feature is very nice, but I would like the drop to be adjustable. 3" Hg is nearly 50% of the standard pressure setting, which I would usually consider too much of a variation in the cure cycle.
The maximum vacuum could probably be increased by switching to larger tubing. At 15" Hg, the neoprene was noticeably constricted, which probably had a large, negative effect on the system conductance (i.e., the resistance went way up). The pump itself accepts standard 3/8" pipe fittings, so larger tubing could be used.
Now, before you start to think that the pump gauges and valves were poorly calibrated, I have to point out that I always have trouble with vacuum measuring devices. My workshop is at 7500' above sea level, where atmospheric pressure is 22.4" Hg, which is a vacuum of 7.5" Hg. The dial gauge on the reservoir reads 0" Hg when it is open to atmosphere, so I don't really know what the pressure was in my system when pumping.
The 15" Hg maximum, plus the 7.5" Hg atmosphere gives an absolute vacuum of 22.5" Hg, close to the advertised capacity of the pump. However, an absolute gauge should read 7.5" Hg in my shop atmosphere. For the 15" Hg + 7.5" Hg to be correct, the gauge would have to be a relative gauge (which I don't think is the case). But then the 10" Hg initial setting would really be 17.5" Hg, which would be significantly different than the claimed setting.
I performed one final pump-down test. I made a tube bag 5 feet long and placed the breather cloth inside. After sealing the bag and starting the pump, it took 1:30 to reach rough vacuum (the dial gauge just started to move off of 0" Hg). It took a total of 2:07 to reach the set value of 10" Hg. The pump then shut off, and I watched for leaks. I noticed a very small leak of less than 0.25" Hg per minute, which is actually very good. With the auto on/off valve, there would be no problem maintaining a good vacuum over a long cure cycle.
In conclusion, this would be a good pump for the hobbyist or as a spare for making small parts in a shop. Because of its small pumping speed, I wouldn't want to use it on very large parts in a production shop, especially where time was important. The combined price for the kit was a good deal, but I would have preferred standard bagging (not tube) and bag sealing tape for my applications.
If anyone else has used this sytem, I would like to hear about your experiences with the gauges. You can e-mail me at composite.guide@about.com. I don't have any way to test the pump at sea level to determine if that makes a difference. Well, I suppose I could put it in an autoclave and watch the gauge through a viewport...
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